Low carryover fittings are required to couple tubing to a device in many applications. For example, a fitting may be used to handle corrosive fluid or gas in an inert manner, or to connect tubing to a blood chemistry analyzer, medical chemical analyzer, DNA research equipment, chromatography equipment, nuclear atomic trace equipment, or other precision fluidic application.
For these applications, the fitting must provide a leak proof connection with substantially zero carryover. Carryover occurs when there is a non-flushable dead volume in a system flow passage or component, which typically occurs when there is an abrupt change in diameter in the passage or component. The dead volume is the quantity of the sample retained inside the passage after flushing with a specified volume. This dead volume may retain material to contaminate subsequent samples or fluids. For the applications listed above, carryover is unacceptable.
As shown in FIG. 11, typical prior art ferrules, such as ferrule 1320 have a tail section 1326 with a blunt end 1326a. Alternatively, tail 1326 may have a rounded corner 1326b. When tubing 1364 is forced over the blunt end 1326 of ferrule 1320, there is a large dead volume 1369, resulting in substantial carryover, even if the passage 1328 is flushed.
A known low carryover fitting for use with tetraflouroethylene (TFE) tubing is the 125 Minstac fitting manufactured by the Lee Company of Westbrook, Conn. The fitting includes an internally threaded collet or sleeve that grips the outer diameter of the tubing end. The end of the tubing is chamfered to receive a ferrule having a cone shaped tail that is received by the chamfer in the tubing. The ferrule has an inner passage that is approximately 0.002 inches less in diameter than the tubing, to provide a low dead volume where the two meet. The ferrule also has a cone shaped head that seats against a chamfered sealing surface in the device. A coupling screw acts like a compression fitting and presses the chamfered end of the tubing against the tail of the ferrule.
Several steps are involved in the use of the Minstac fitting. In order to use the fitting, a special tool called a collet tool is needed to crimp the collet onto the tubing. A second tool, called a chamfer tools is used to chamfer the tubing. This may be particularly difficult with slippery tube materials such as TFE. In order to provide sufficient compression to form a seal between the tubing and the device, without damaging the tubing or the ferrule, the coupling screw must be tightened within a limited range of torque. Typically a special torque wrench is used to tighten the coupling screw. If excessive torque is applied, the ferrule may push the tubing out of the collet. Furthermore, soft tubing materials, such as TFE composites, can deform inward, so that the collet may not grip them securely enough.
A low carryover fitting is desired that is simple to install and requires no special tools. It is also desired that the fitting includes a mechanism to prevent damage due to excessive torque.